Inadequate drainage of surplus waters ruins soils, keeps ground-water tables injuriously high and causes standing, stagnant water, or local flooding. Uncontrolled discharge of drainage waters can result in the inundation of the territory of downstream countries. Large tracts of land are agriculturally unproductive because of excess water in or on the soil, due to inadequate land drainage. Water in excess of that needed by plants may inhibit growth of the economically important parts of the plant. High water content also lubricates the soil particles and frequently leads to unstable conditions unsuitable to machine and other crop operations. In the extreme, when land is drowned, waterlogged or marshy, it may be of no value to crop production. Excess water creates problems in agricultural production over vast areas.
There are no figures to indicate the scale of the problem on a world wide basis, but it has been estimated that, in the USA alone, about a quarter of agricultural land would benefit from land drainage. Inadequate drainage on irrigated lands presents a different facet of the same problem. Badly drained irrigated lands can become marshy or salty and have to be abandoned. There are throughout the world 20-25 million hectares of now saline and barren lands which were once fertile. The loss of irrigated lands to agriculture is continuing, and every year between 200,000 and 300,000 hectares of irrigated lands fall victim to salinization or revert to marshland.